1. Field of the Invention
The present invention relates to a transmission type screen for rear projection type image display devices in which an image from a projection device is projected from the rear surface of the screen, more specifically, to a transmission type screen suitable for rear projection type image display devices provided with a projection device of a single-lens system using an image panel having a plurality of pixels, such as liquid-crystal panels and digital micromirror devices (referred to as DMD hereinbelow).
2. Description of the Related Art
CRTs of a red (R), green (G), and blue (B) three-tube system that feature the possibility of obtaining a high brightness have been usually used as image projectors in rear projection type display devices. Transmission type screens used in combination with the three-tube CRTs are composed of a lenticular lens sheet and a Fresnel lens sheet. In a lenticular lens sheet 11, as shown schematically in FIG. 7, black stripes 13 are formed in positions different from a converging position of a light beam incident from lenses 12 located on the light incidence side. Herein, the lens pitch in the lenticular lens sheet was usually no less than 0.5 mm and the sheet thickness was usually no less than 0.6 mm in order to obtain a sufficient contrast, a wide-range view angle and also a high mechanical strength.
However, CRTs of a red (R), green (G), and blue (B) three-tube system were difficult to make compact and lightweight, while maintaining a sufficient brightness. Accordingly, in recent years, rear projection type image display devices have been developed in which the weight and size of projection devices were decreased by using a liquid-crystal panel, DMD, and the like as the projection devices. Liquid-crystal panels or DMD have a large number of pixels, and when such projection devices were used in combination with screens that have been used in the conventional three-tube CRT projection devices, because both the pixels of the liquid-crystal panel, DMD, and the like and the lenses of the lenticular lens sheet have a periodic structure in the horizontal direction, a problem was associated with the appearance of moirxc3xa9 formed by the pixels and lenticular lenses.
A known technology (see Japanese Patent Application Laid-open No. H02-97991) avoids the appearance of such moirxc3xa9 by employing the ratio of the lenticular lens pitch to the horizontal pitch of pixel images projected on the screen of 1/(N+xc2xd) (N is integer of no less than 1). Usually, an N value of no less than 2 is used. For example, if the pitch of pixel images projected on the screen is assumed to be 1.0 mm, the pitch of lenticular lenses satisfying the above-mentioned condition will be as small as 0.4 mm when N is 2, 0.28 mm when N is 3, and 0.222 mm when N is 4. In recent years the pitch of lenticular lenses set according to such technology tended to decrease because resolution of screens was increasing and the pitch of pixel images was decreasing.
The conventional lenticular lens sheets have been optically designed so that a light beam incident on the lens on the light incidence side was focused in a point outside the lens on the exit side, as shown in FIG. 8 (see Japanese Patent Application Laid-open S58-221833). In the lenticular lens sheet thus designed, the ratio Lt/LP1 of the maximum distance from the lens surface on the light incidence side of a lenticular lens sheet to the exit surface (Lt, referred to as xe2x80x9cthe thickness of a lenticular lens sheetxe2x80x9d hereinbelow) to the pitch of lenticular lenses (LP1) is typically within a range of 1.1xcx9c1.25 to guarantee the desired characteristics of the screen in the horizontal direction. For example, when the pitch of lenticular lenses is 0.72 mm, the thickness of the lenticular lens sheet is 0.87 mm. With the thickness of the lenticular lens sheet of such order, the mechanical strength is sufficient and the sheet can be easily manufactured. On the other hand, in order to eliminate moirxc3xa9 in accordance with the invention disclosed in Japanese Patent Application Laid-open No. H02-97991, if the pitch of lenticular lenses is 0.22 mm, the thickness of a lenticular lens sheet becomes as small as 0.242xcx9c0.275 mm. The lenticular lens sheets are usually fabricated by using optically transparent thermosetting resins. If the thickness of a lenticular lens sheet is as small as 0.242xcx9c0.275 mm, the sheet is difficult to manufacture and the mechanical strength thereof is low.
Accordingly, if the thickness (Lt) of a lenticular lens sheet is no less than 0.3 mm and the pitch (LP1) of lenticular lenses is 0.22 mm in order to facilitate the manufacture and to obtain a sufficient mechanical strength, the ratio Lt/LP1 thereof will be no less than 1.36. For example, when the pitch of lenticular lenses is 0.22 mm and the thickness of a lenticular lens sheet is 0.87 mm, if the shape of the lens 3 on the light incidence side is determined so as to obtain a sufficient horizontal diffusion characteristic, the light beam is sometimes shielded by the surface of black stripes 2, as shown in FIG. 9, and the loss of quantity of light sometimes becomes too large.
On the other hand, if the shape of lenses 3 on the light incidence side is designed so that the light beam is shielded by the surface of black stripes 2, then, as shown in FIG. 10, it becomes necessary to almost match the focal distance of lenses 3 on the light incidence side with the sheet thickness and a sufficient horizontal diffusion characteristic cannot be obtained.
As described above, the problem associated with the prior art technology was in that it was difficult to obtain a lenticular lens sheet with a small pitch which was easy to manufacture and which had a high mechanical strength and also demonstrated a high diffusion performance.
In view of the foregoing, it is an object of the present invention to provide a transmission type screen which makes it possible to obtain an excellent horizontal diffusion characteristic, to eliminate moirxc3xa9, and to increase contrast.
According to one aspect of the present invention, for achieving the above-mentioned object, there is provided a transmission type screen comprising a Fresnel lens sheet, a lenticular lens sheet provided with convex black stripes on the light-exit surface, and a diffusion sheet having an uneven surface on one side thereof, wherein the uneven surface of the diffusion sheet is joined to the lenticular lens sheet at the top portions of black stripes.
Consequently, it is possible to obtain an excellent horizontal diffusion characteristic, to eliminate moirxc3xa9, and to increase contrast.
The above and further objects and novel features of the invention will more fully appear from the following detailed description when the same is read in connection with the accompanying drawings. It is to be expressly understood, however, that the drawings are for purpose of illustration only and are not intended as a definition of the limits of the invention.